These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Franchise Original Sin: For two factors. One for creating the decision to release the series over multiple consoles, creating Continuity Lock-Out (an act somewhat rectified by the HD compliations). Two for introducing a lot of new characters for the bad guys, taking away focus on the Disney and Final Fantasy elements that brought people in.
The Duel system in Re:Chain of Memories, especially Barrage. This attack will always hit and always do 1 bar damage. It's possible to beat Ansem easily without Dark Mode because of this.
Certain sleights can make the game very easy, if you build your deck right. With enough 0s, you can't be cancelled, and a ton of Sonic Blade + Elixir = Win. Once you have the Jafar card this makes using sleights like Sonic Blade even more deadly. A deadly pair to use would be Jafar with Ars Arcanum. Normally this sleight would require 3 cards from 1-6 in value, very breakable. If Jafar is used before, the sleight becomes unbreakable. This makes most bosses become a breeze.
The Tornado Sleight (Aero + Gravity + Summon card) is absolutely unfair in both versions of the game: it creates a large funnel cloud that the player can control, which sucks up and immobilizes all Heartless it touches while dealing massive damage.
The Oogie Boogie card (Regen) quickly becomes one of the best buff cards, especially against tough bosses.
The Lethal Frame (mistranslated as "Lethal Flame") sleight (Stop + Attack + Attack) stops time and allows Sora to quickly deliver a bunch of hits. When time starts again, the enemy starts taking damage and can't act for a few moments, allowing you to set up another one. Long strings of these can deal massive damage in boss fights.
Simply using 0's to Card Break your opponent's deck. Every card you break with 0 can't be reshuffled by the boss, while you can recharge your deck indefinitely. After awhile, the boss will simply begin to run out of cards to make combos with, making him easy prey.
The Jafar Card in the GBA version. As mentioned on the main page under Boring but Practical, it makes your attacks immune to breaking so long as you attack first. The thing about the GBA version, however, is that the effect was limited was 20 attacks, not cards like the later versions. That means sleights don't count, so you can use risky low-value sleights like Ars Arcanum without being broken, and keep using them throughout the whole battle.
The remake introduces enemy cards based off the Organization. Xemnas in particular stands out with Quick Barrier, which lasts throughout most if not all of your typical battle and cuts most damage received in half as its secondary effect. Its primary effect is to prevent Sora from taking damage more than once from a given card, and since most bosses use attacks that hit multiple times, this effectively neuters their offense. Like Sonic Blade, it can be gotten about halfway through the game.
Genius Bonus: In the manga adaptation of CoM, Larxene is seen reading a book about French writer Marquis De Sade. We'll let you do the math.
Any enemy you have to attack from behind in the Game Boy Advance game. This is remedied a bit in Re:CoM because you're able to attack from more angles - in the original, it's roughly 2D so you have to be right behind them. Defenders are easily the most annoying mooks in the game.
The Creeper Plants have a terrible tendency to break your cards.
Neoshadows. They also have a tendency to turn two-dimensional and avoid everything you throw at them. This is especially annoying because they seem to wait right until you use a sleight.
Good Bad Bugs: The "Double jump glitch" in the Game Boy Advance version. When you double-jump (B twice) and perform an attack, Riku will jump down at his enemy and hit them numerous times... but if you have an attack modifier like Overdrive, Berserk, or Attack Bracer on, it doesn't take down the charges and this attack still has the effects of the modifier.
Vexen's shockingly brutal death was mitigated somewhat by the "Can't feel real emotions" tidbit from Kingdom Hearts II, but then 3D: Dream Drop Distance reveals nobodies regrow hearts and can experience emotions, it becomes just as bad if not worse than it first was, because Vexen was legitimately terrified at the thought of dying.
Hilarious in Hindsight: Larxene's voice actor, Shanelle Gray, also narrates some of the Wendy's commercials. Then, during the cutscene just before fighting her a second time,
Larxene: Okay... Have it your way. More pain for you means more fun for me!
Paranoia Fuel: You know that big crystal ball in the room where Marluxia, Larxene, and Axel talk about their plans? They use it to see everything going on in Castle Oblivion. So the three of them are watching everything you do throughout the whole game.
Narm: While Marluxia dies, he has this extremely obnoxious yell that makes it sound like he's taking the biggest crap of his life, which tends to make people laugh while he dies.
Replacement Scrappy: Naminé was this to some, although it got better as more people played the game. And even more when it was discovered in Kingdom Hearts II that she's Kairi's Nobody.
Surprise Difficulty: The Bumble-Rumble fight in the 100-Acre Wood. The goal is to fight off 50 swarms of bees without letting Pooh's HP run out. This is, unfortunately, obnoxiously difficult due to being stuck with a fixed Deck mostly comprised of middle to low numbers and the bees coming in packs and carrying cards as high as 8. Your only reprieve is that you can use any Sleight you've already learned, plus two special Sleights that involve the exclusive Hunny card.
The final battle against the Riku Replica (in both Sora AND Riku's modes), so freaking much.
Trickmaster in Reverse/Rebirth. Riku's deck in Wonderland is not good, having nothing higher than 5. This also makes it very difficult to break anything Trickmaster throws at you without stocking cards.
Any time you fight Vexen in the original Chain of Memories. You have to attack him from behind - this is harder in the original than it is in the remake. He also has a tendency to turn right around and attack you when you try to hit him.
Vexen also has a one-time use Enemy-type card that, when HP goes to 0, revives him with a quarter of his HP restored. Yeah, you thought you were finally finished with him, didn't you? Thankfully, it's also his drop loot, so you can acquire and use it afterward.
That isn't to say that Re:CoM Vexen is easy. His attacks are pretty hard to dodge and can stunlock, he still can't be attacked from the front and his deck is stacked with high-numbered attack-cards. He also has Elixirs and Hi-Ethers with him, making the strategy of breaking all his good cards hard to execute - especially in the rematch, in which he also owns a card that prevents breaking items.
Captain Hook. His Geo Effects screw up your summon cards, his dodge rate is insane, he can throw bombs seemingly without end, and his sword attack is extremely difficult to dodge. Better bring your zeroes and your cures - you're gonna NEED it all. This review says it all:
You may think that all the bosses before Captain Hook might have been annoying but otherwise easy. You might have thought that the rest of the game would go along like this. Even if that wasn't the case, even if you thought that there would be at least one tough boss along the way, nothing could prepare you for the brokenness that is Captain Hook.
Hades is a good example of this trope as well. He is one of the only Disney bosses (Hook above being the other one) that uses sleights, and loves making your life a living hell with them, which hit like a truck.
What an Idiot: All Jiminy writes in his Journal at the end of Chain of Memories is "Thank Naminé" with no explanation whatsoever. Cue the mass confusion for Sora & co. at the beginning of KHII. Considering that this was fairly important to them, you'd think the note would be a bit more thorough.
The Woobie: Naminé, Kairi's Nobody, who is confined by the Organization to the top of Castle Oblivion for the most part, is forced to do their bidding, and is perfectly aware of her status as a temporary being.